It is well known that hydroquinone is generally purified by extraction and recrystallization, is composed of colorless needle or prismatic crystals, has a melting point of 169.degree. C, has a boiling point of 287.degree. C/73 mm Hg, sublimes at 285.degree. C without decomposition, is soluble in water, ethanol and ether, and is sparingly soluble in cold benzene. Hydroquinone is used as a photographic developing agent, as an intermediate for dyes, organic synthesis, and medicine, or as an oxidation or polymerization inhibitor.
Even when crude hydroquinone, obtained by decomposing p-dialkylbenzene-bis-hydroperoxide (for example, p-diisopropylbenzene-bis-hydroperoxide, p-di-sec. butylbenzene-bis-hydroperoxide, etc.) in an acid medium is purified, utilizing said properties of hydroquinone, for example by recrystallization from water, the resulting purified hydroquinone undergoes coloring or discoloring to a light pink, etc. at least for about one week after the purification, even if the purification is carried out satisfactorily. This may lower the usefulness and value of the hydroquinone when it is used for certain applications, and especially when developing an important photographic picture which cannot be taken again. As a consequence, hydroquine is sometimes not used because of the uncertainties about the possible adverse effects of its occassional undesirable properties.
Therefore, it is very important from the standpoint of product value and reliability to insure that there will be no coloring or discoloring of the hydroquinone and that the hydroquinone will always be in the form of colorless crystals. In the production of hydroquinone, especially according to the cumene process, considerable coloring of the product takes place. For example, the hydroquinone in the decomposition product solution resulting from the decomposition of p-dialkylbenzene-bis-hydroperoxide in an acidic medium undergoes rapid change, especially in the presence of impurities, turning the product to a dark color. It is difficult to obtain a product that can meet the color requirements and other standards of the ASA (American Standards Association) relative to photographic materials, even though it is recrystallized from water. In order to solve this problem it has been proposed to add p-dialkylbenzene-bis-hydroperoxide to a mixture consisting of (a) an aromatic hydrocarbon, non-aqueous solvent capable of dissolving by-products formed in the course of the decomposition, but which is incapable of dissolving hydroquinone formed, and (b) an acid catalyst and dissolving the impurities during the decomposition and suppressing side reactions of hydroquinone with the impurities throughout the entire process of forming the hydroquinone, while placing the hydroquinone outside the solvent, and recovering the hydroquinone insoluble in the aromatic hydrocarbon solvent from said solvent by a mechanical means such as filtration (see Japanese Patent Publication No. 1538/1967).
It is well known, as described above, that it is difficult to improve the hydroquinone of not-so-good quality itself up to the desired quality even by the recrystallization of the hydroquinone from water, etc. Furthermore, it is difficult to completely prevent the coloring or discoloring of the hydroquinone in spite of the fact that the hydroquinone produced according to the cumene process is properly colorless, even though the said prior art process should be carried out. Thus, it is obviously very preferable in view of the product value to prevent the coloring or discoloring completely.